In an optical transmission network, a double transmission path that couples optical transmission devices is provided. In an optical protection scheme, when a failure has occurred in one of transmissions path (work system), the transmission path is switched to the other transmission path (protection system), so that optical transmission continues. In an optical unidirectional path-switched ring (OUPSR) scheme that is an optical protection scheme, a transmission-side transponder device transmits the same optical signal to both of the work system and the protection system, and a reception-side transponder device switches between the work system and the protection system using an optical switch or the like. The reception-side transponder device transfers the optical signal to a subsequent client device. The light receiving properties of the transponder devices differ in accordance with the level of the optical signal received from the optical transmission network, for example, the light level. Variation in the level of input light markedly affects the receiving properties of the transponder devices at an optical transmission speed of about 10 Gbps.
Related art is disclosed in International Publication Pamphlet No. WO 2006/035481 and Japanese Laid-open Patent Publication No. 2005-12689.